16-01-2013, 04:29 PM
David, there are several types of switched mode supplies of varying complexities. The cheap Chinese junk type (laptop/phone/etc.) are honestly not worth repairing and you sometimes have to destroy the case to get it apart.
However, this sounds like a 'middle of the road' item which should have protection circuitry in the event of component failure. Normally, the mains supply is full-wave rectified and applied to the 450V electrolytic capacitor to provide a DC source for the rest of the unit. I suggest you locate the bridge rectifier that feeds that capacitor and check that both arms are still ok as you need full wave for the PSU to work.
Also check for a blown fuse on the pcb.
If you replace the capacitor, it will need to be a high ripple current rated type.
It would also checking the power device on the heatsink for short circuits. They normally go S/C and not O/C.
The dry joint(s) suggests this may not be a high quality item. It would be worth inspecting the rest of the board for similar joints. I've found them where the holes for the wound components are to big for the wires to allow for differences in dimensions and the expansion and contraction of the paxolin board couple with the mass of the wound component eventually leads to dry joints. This is especially true if the pin in exactly in the middle of the hole with an air space around it. They all use jigs these days, so it shouldn't be difficult to make the holes in the board fit the components correctly.
When happy, perhaps you should power it up with a dummy resistive load before taking voltage measurements as these supplies often need a load to regulate and won't work under open circuit conditions. If you take 100mA from the supply (12V bulb ?) that should be fine.
However, this sounds like a 'middle of the road' item which should have protection circuitry in the event of component failure. Normally, the mains supply is full-wave rectified and applied to the 450V electrolytic capacitor to provide a DC source for the rest of the unit. I suggest you locate the bridge rectifier that feeds that capacitor and check that both arms are still ok as you need full wave for the PSU to work.
Also check for a blown fuse on the pcb.
If you replace the capacitor, it will need to be a high ripple current rated type.
It would also checking the power device on the heatsink for short circuits. They normally go S/C and not O/C.
The dry joint(s) suggests this may not be a high quality item. It would be worth inspecting the rest of the board for similar joints. I've found them where the holes for the wound components are to big for the wires to allow for differences in dimensions and the expansion and contraction of the paxolin board couple with the mass of the wound component eventually leads to dry joints. This is especially true if the pin in exactly in the middle of the hole with an air space around it. They all use jigs these days, so it shouldn't be difficult to make the holes in the board fit the components correctly.
When happy, perhaps you should power it up with a dummy resistive load before taking voltage measurements as these supplies often need a load to regulate and won't work under open circuit conditions. If you take 100mA from the supply (12V bulb ?) that should be fine.
